Why is my Eevee viewport render different from my full render?
To quote an answer I got elsewhere,


  What makes the rendered image look dark is the volume material of the
  particle emitter. However, the emitter is not shown because you have
  to enable “show emitter” in the particle options :slight_smile:
  Unfortunately, the “show emitter” checkbox doesnt have influece on the
  viewport so you always need to double check if it’s turned on or off.
   --eaNiiX